Crash sensors for determining that a vehicle is in a crash of sufficient magnitude as to require the deployment of an inflatable restraint system, or airbag, are either mounted in a portion of the front of the vehicle which has crushed by the time that sensor triggering is required, the crush zone, or elsewhere such as the passenger compartment, the non-crush zone. Regardless of where sensors are mounted there will always be crashes where the sensor triggers late and the occupant has moved to a position near to the airbag deployment cover. In such cases, the occupant may be seriously injured or even killed by the deployment of the airbag. This invention is largely concerned with preventing such injuries and deaths by preventing late airbag deployments.
In a Society of Automotive Engineers (SAE) paper by Mertz, Driscoll, Lenox, Nyquist and Weber titled "Response of Animals Exposed to Deployment of Various Passenger Inflatable Restraint System Concepts for a Variety of Collision Severities and Animal Positions" SAE 826074, 1982, the authors show that an occupant can be killed or seriously injured by the airbag deployment if he or she is located out of position near or against the airbag when deployment is initiated. These conclusions were again reached in a more recent paper by Lau, Horsch, Viano and Andrzejak titled "Mechanism of Injury From Air Bag Deployment Loads", published in Accident Analysis & Prevention, Vol 25, No.1, 1993, Pergamon Press, New York, where the authors conclude that "Even an inflator with inadequate gas output to protect a properly seated occupant had sufficient energy to induce severe injuries in a surrogate in contact with the inflating module." These papers highlight the importance of preventing deployment of an airbag when an occupant is out of position and in close proximity to the airbag module.
The Ball-in-Tube crush zone sensor, such as disclosed in U.S. Pat. Nos. 4,974,350; 4,198,864; 4,284,863; 4,329,549; 4,573,706 and 4,900,880 to D. S. Breed, has achieved the widest use while other technologies, including magnetically damped sensors as disclosed in U.S. Pat. No. 4,933,515 to Behr et al and crush switch sensors such as disclosed in U.S. Pat. No. 4,995,639 to D. S. Breed, are now becoming available. Other sensors based on spring-mass technologies are also being used in the crush zone. Crush zone mounted sensors, in order to function properly, must be located in the crush zone at the required trigger time during a crash or they can trigger late. One example of this was disclosed in a Society of Automotive Engineers (SAE) Paper by Breed and Castelli titled "Trends in Sensing Frontal Impacts", SAE 890750, 1989, and further in U.S. Pat. No. 4,900,880. In impacts with soft objects, the crush of a vehicle can be significantly less than for impacts with barriers, for example. In such cases, even at moderate velocity changes where an airbag might be of help in mitigating injuries, the crush zone mounted sensor might not actually be in the crush zone at the time that sensor triggering is required for timely airbag deployment, and as a result can trigger late when the occupant is already resting against the airbag module.
There is a trend underway toward the implementation of Single Point Sensors (SPS) which are typically located in the passenger compartment. In theory, these sensors use sophisticated computer algorithms to determine that a particular crash is sufficiently severe as to require the deployment of an airbag. In another SAE paper by Breed, Sanders and Castelli titled "A Critique of Single Point Sensing", SAE 920124, 1992, which is included herein by reference, the authors demonstrate that there is insufficient information in the non-crush zone of the vehicle to permit a decision to be made to deploy an airbag in time for many crashes. Thus, sensors mounted in the passenger compartment or other non-crush zone locations, will also trigger the deployment of the airbag late on many crashes.
A crash sensor is necessarily a predictive device. In order to inflate the airbag in time, the inflation must be started before the full severity of the crash has developed. All predictive devices are subject to error, so that sometimes the airbag will be inflated when it is not needed and at other times it will not be inflated when it could have prevented injury. The accuracy of any predictive device can improve significantly when a longer time is available to gather and process the data. One purpose of the occupant position sensor is to make possible this additional time in those cases where the occupant is farther from the steering wheel when the crash begins and/or where, due to seat belt use or otherwise, the occupant is moving toward the steering wheel more slowly. In these cases the decision on whether to deploy the airbag can be deferred and a more precise determination made of whether the airbag is needed.
The discussions of timely airbag deployment above are all based on the seating position of the average male (the so called 50% male) relative to the airbag or steering wheel. For the 50% male, the sensor triggering requirement is typically calculated based on an allowable motion of the occupant of 5 inches before the airbag is fully inflated. Airbags typically require about 30 milliseconds of time to achieve full inflation and, therefore, the sensor must trigger inflation of the airbag 30 milliseconds before the occupant has moved forward 5 inches. The 50% male, however, is actually the 70% person and therefore about 70% of the population sit on average closer to the airbag than the 50% male and thus are exposed to a greater risk of interacting with the deploying airbag. A recent informal survey, for example, found that although the average male driver sits about 12 inches from the steering wheel, about 2% of the population of drivers sit closer than 6 inches from the steering wheel and 10% sit closer than 9 inches. Also, about 1% of drivers sit at about 24 inches and about 16% at least 18 inches from the steering wheel. None of the sensor systems now on the market take account of this variation in occupant seating position and yet this can have a critical effect on the sensor required maximum triggering time.
For example, if a fully inflated airbag is about 7 inches thick, measured from front to back, then any driver who is seated closer than 7 inches will necessarily interact with the deploying airbag and the airbag probably should not be deployed at all. For a recently analyzed 30 mph barrier crash of a mid-sized car, the sensor required triggering time, in order to allow the airbag to inflate fully before the driver becomes closer than 7 inches from the steering wheel, results in a maximum sensing time of 8 milliseconds for an occupant initially positioned 9 inches from the airbag, 25 milliseconds at 12 inches, 45 milliseconds at 18 inches and 57 milliseconds for the occupant who is initially positioned at 24 inches from the airbag. Thus for the same crash, the sensor required triggering time varies from a no trigger to 57 milliseconds, depending on the initial position of the occupant. A single sensor triggering time criterion that fails to take this into account, therefore, will cause injuries to small people or deny the protection of the airbag to larger people. A very significant improvement to the performance of an airbag system will necessarily result from taking the occupant position into account as described herein.
A further complication results from the fact that a greater number of occupants are now wearing seatbelts which tends to prevent many of these occupants from getting too close to the airbag. Thus, just knowing the initial position of the occupant is insufficient and either the position must be continuously monitored or the seatbelt use must be known. Also, the occupant may have fallen asleep or be unconscious prior to the crash and be resting against the steering wheel. Some sensor systems have been proposed that double integrate the acceleration pulse in the passenger compartment and determine the displacement of the occupant based on the calculated displacement of an unrestrained occupant seated at the mid seating position. This sensor system then prevents the deployment of the airbag if, by this calculation, the occupant is too close to the airbag. This calculation can be greatly in error for the different seating positions discussed above and also for the seatbelted occupant, and thus an occupant who wears a seatbelt could be denied the added protection of the airbag in a severe crash.
As the number of vehicles which are equipped with airbags is now rapidly increasing, the incidence of late deployments is also increasing. It has been estimated that out of approximately 400 airbag related complaints to the National Highway Traffic Safety Administration (NHTSA) through 1991, for example, about 5% to 10% involved burns and injuries which were due to late airbag deployments. There is also at least three known fatalities where a late airbag deployment is suspected as the cause.
The need for an occupant position sensor has been observed by others and several methods have been disclosed in U.S. patents for determining the position and velocity of an occupant of a motor vehicle. Each of these systems, however, have significant limitations. In White et al U.S. Pat. No. 5,071,160, for example, a single acoustic sensor and detector is disclosed and illustrated mounted lower than the steering wheel. White et al correctly perceive that such a sensor could be defeated, and the airbag falsely deployed, by an occupant adjusting the control knobs on the radio and thus they suggest the use of a plurality of such transmitter/receivers. If a driver of a vehicle is seated one foot from the transmitter/receiver, and using 1128 feet per second as the velocity of sound, it would require approximately 2 milliseconds for the sound to travel to the occupant and return. The use of the same device to both transmit and detect the sound waves requires that the device cannot send and receive simultaneously and therefore it requires at least 2 milliseconds to obtain a single observation of the occupant's position. Naturally as the distance from the occupant to the sensor increases, the observation rate further decreases. For a passenger sitting two feet from the sensor, the delay is approximately 4 milliseconds. Sensors of this type can be used to accurately obtain the initial position of the occupant but the determination of the occupant's velocity, and thus the prediction of when he/she is likely to be too close to the deploying airbag, will necessarily be inaccurate due to the long delay between position points and thus the small number of such points available for the prediction and the inherent noise in the reflected signal.
Mattes et al in U.S. Pat. No. 5,118,134 disclose a single ultrasonic transmitter and a separate receiver, but, no description is provided as to the manner in which this combination is used. In conventional ultrasonic distance measuring systems, the transmitter emits a burst of ultrasonic waves and then measures the time required for the waves to bounce off the object and reach the receptor. The transmitter does not transmit again until the waves have been received by the receiver. This system again suffers from the time delay of at least 2 to 4 milliseconds described above.
Doppler techniques can be used to determine the velocity of the occupant as disclosed below. Both White et al and Mattes et al, however, specifically state that the occupant's velocity is determined from a succession of position measurements. The use of the Doppler effect is disclosed in U.S. Pat. No. 3,275,975 to King, but only to determine that the occupant is not moving. No attempt is made by King to measure the velocity of the occupant toward an airbag using this effect. Also none of the references above disclose the use of an ultrasonic transmitter and receiver to simultaneously determine the position and velocity of the occupant using a combination of the transmission time and the Doppler effect as disclosed below.
The object of an occupant position sensor is to determine the location of the head and/or chest of the vehicle occupant relative to the airbag since it is the impact of either the head or chest with the deploying airbag which can result in serious injuries. For the purposes herein, therefore, whenever the position of the occupant is referenced it will mean the position of the head or chest of the occupant and not that of his/her arms, hands or legs. The preferred mounting of the ultrasonic transmitters, therefore, are those locations which have the clearest unimpeded view of the occupant's head and chest. These locations are generally at the top of the dashboard, the windshield, the headliner above the windshield and the rear view mirror. Both White et al and Mattes et al disclose only lower mounting locations of the ultrasonic transmitters such as on the dashboard or below the steeling wheel. Both such mounting locations are particularly prone to detection errors due to positioning of the occupant's hands, arms and legs. This would require at least three, and preferably more, such sensors and detectors and an appropriate logic circuitry for the case where the driver's arms are the closest objects to two of the sensors. When an unimpeded view is not possible, some means of pattern recognition, which is not disclosed in the above references, is required to differentiate between the occupant and his/her extremities such as his/her hands, arms or legs.
Mattes et al further disclose the placement of the sensor in the headrest but such an arrangement is insufficient since it measures the distance from the headrest to the occupant and not from the airbag.
White et al discloses the use of error correction circuitry to differentiate between the velocity of one of the occupant's hands as in the case where he/she is adjusting the knob on the radio and the remainder of the occupant. Three ultrasonic sensors of the type disclosed by White et al would accomplish this differentiation if two of them indicated that the occupant was not moving while the third was indicating that he or she was. Such a combination, however, would not differentiate between an occupant with both hands and arms in the path of the ultrasonic transmitter at such a location that it was blocking a substantial view of the occupant's head or chest. Since the sizes and driving positions of occupants are extremely varied, pattern recognition systems are required when a clear view of the occupant, unimpeded by his/her extremities, cannot be guaranteed. Pattern recognition systems for the occupant as used here means any system which will differentiate between the occupant and his extremities based on relative size, position or shape. Pattern recognition systems can also be used to differentiate an occupant from a seat or a bag of groceries also based on relative size, position or shape or even on passive infrared radiation, as described below.